Commercial, or civil, aircraft and industrial land based gas turbines are conventionally designed for reducing exhaust emissions from combustion of hydrocarbon fuel such as, for example, Jet-A fuel. The exhaust emissions may include hydrocarbon particulate matter, in the form of smoke, for example, gaseous hydrocarbons, carbon monoxide, and nitrogen oxide (NO.sub.x) such as, for example nitrogen dioxide NO.sub.2. NO.sub.x emissions are known to occur from combustion at relatively high temperatures, for example over 3,000.degree. F. (1648.degree. C.). These temperatures occur when fuel is burned at fuel/air ratios at or near stoichiometric, or, alternatively, at or near an equivalence ratio of 1.0, which represents actual fuel/air ratio divided by the stoichiometric fuel/air ratio. The amount of emissions formed is directly related to the time, i.e., residence time, that combustion takes place at these conditions.
Conventional gas turbine engine combustors for use in an engine for powering an aircraft, naval vessel, or power plant are conventionally sized and configured for obtaining varying fuel/air ratios during the varying power output requirements of the engine such as, for example, lightoff, idle, takeoff, and cruise modes of operation of the engine in the aircraft. At relatively low power modes, such as lightoff and idle, a relatively rich fuel/air ratio is desired for initiating combustion and maintaining stability of the combustion. At relatively high power modes, such as for example cruise operation of the engine in the aircraft, a relatively lean fuel/air ratio is desired for obtaining reduced exhaust emissions.
Although stoichiometric fuel/air mixtures are preferred for obtaining substantially complete combustion, it is known that NO.sub.x emissions are maximized at the stoichiometric condition and are reduced for both rich and lean operation since NO.sub.x formation increases with increased flame temperature, as well as with increased residence time. In order to reduce NO.sub.x emissions, rich-lean staged combustors are known which may either be radially or axially staged. In the axial rich-lean staged combustor, a rich stage is provided for first burning rich fuel/air mixtures, with incomplete combustion thereof, which is then rapidly quenched with dilution air to a lean equivalence ratio. Combustion is then completed in the lean stage at relatively low temperature for reducing the NO.sub.x emissions.
However, since the quenching occurs in a relatively short axial length, it is difficult to adequately mix the rich, partially burned mixture with the dilution air without providing pockets of stoichiometric mixtures therein which will generate NO.sub.x emissions. Mixing is also required to obtain a good pattern factor, i.e., uniform temperature profile at the combustor outlet. A typical axially staged rich-lean combustor has an hour glass shape with a reduced area to accelerate the flow for reducing residence time as well as introducing dilution air for quenching. Other configurations of axially and radially staged rich-lean combustors vary in complexity. And, conventional combustors typically used in an aircraft gas turbine engine are more simply configured with radially spaced apart, annular, outer and inner combustion liners defining an annular (annulus) combustion chamber therebetween, but are not effective for rich-lean staged operation.